Enable debug-only PowerShell Editor Services sessions

This change finishes the work necessary to enable debug-only sessions
where a new PowerShell process is launched strictly for the purpose of
providing a fresh debugging session.  This is useful when debugging
modules which use PowerShell classes or binary components which cannot
be reloaded in the same process.

Part of the implementation for PowerShell/vscode-powershell#367.

Author: daviwil

module/PowerShellEditorServices/PowerShellEditorServices.psm1

@@ -49,7 +49,7 @@ function Start-EditorServicesHost {
         [switch]
         $EnableConsoleRepl,
 
-        [string]
+        [switch]
         $DebugServiceOnly,
 
         [string[]]

src/PowerShellEditorServices.Host/EditorServicesHost.cs

@@ -256,44 +256,44 @@ private void OnDebugServiceClientConnect(object sender, TcpSocketServerChannel s
 
             protocolEndpoint.UnhandledException += ProtocolEndpoint_UnhandledException;
 
-            if (this.enableConsoleRepl)
-            {
-                this.debugAdapter =
-                    new DebugAdapter(
-                        this.editorSession,
-                        false,
-                        messageDispatcher,
-                        protocolEndpoint,
-                        this.logger);
-            }
-            else
+            bool ownsEditorSession = this.editorSession == null;
+
+            if (ownsEditorSession)
             {
-                EditorSession debugSession =
+                this.editorSession =
                     this.CreateDebugSession(
                         this.hostDetails,
                         profilePaths,
                         protocolEndpoint,
                         messageDispatcher,
                         this.languageServer?.EditorOperations,
-                        false);
-
-                this.debugAdapter =
-                    new DebugAdapter(
-                        debugSession,
-                        true,
-                        messageDispatcher,
-                        protocolEndpoint,
-                        this.logger);
+                        this.enableConsoleRepl);
             }
 
+            this.debugAdapter =
+                new DebugAdapter(
+                    this.editorSession,
+                    ownsEditorSession,
+                    messageDispatcher,
+                    protocolEndpoint,
+                    this.logger);
+
             this.debugAdapter.SessionEnded +=
                 (obj, args) =>
                 {
-                    this.logger.Write(
-                        LogLevel.Normal,
-                        "Previous debug session ended, restarting debug service listener...");
-
-                    this.debugServiceListener.Start();
+                    if (!ownsEditorSession)
+                    {
+                        this.logger.Write(
+                            LogLevel.Normal,
+                            "Previous debug session ended, restarting debug service listener...");
+
+                        this.debugServiceListener.Start();
+                    }
+                    else
+                    {
+                        // Exit the host process
+                        this.serverCompletedTask.SetResult(true);
+                    }
                 };
 
             this.debugAdapter.Start();